Magnetic resonance imaging (MRI) is an imaging technique often used in medical diagnostic applications in order to create detailed images of internal organs and internal physical features that cannot be obtained otherwise. For example, MRI may be used to image and diagnose neurological and cardiovascular diseases, as well as to assess the function and health of organs such as the prostate, liver, pancreas, and bile ducts. For oncology applications, as an illustrative example, T2-weighted MRI has been investigated for prostate cancer detection, but currently requires highly-qualified subspecialty radiologists to interpret the data due to its weak delineation between cancerous tissue and healthy tissue.
Diffusion weighted imaging (DWI) is a type of MRI in which pairs of opposing magnetic field gradient pulses are applied to obtain sensitivity to the Brownian motion of water molecules in tissues. The differences in diffusion characteristics between tissue types facilitate tissue characterization. As an example, DWI may be used for diagnosing prostate cancer which has a presumed high cellular density relative to the surrounding tissue. The cancerous tissues should therefore exhibit restricted diffusion characteristics (and as such should have lower apparent diffusion coefficient (ADC) values) relative to the surrounding tissue. While DWI is a promising imaging modality for diagnostic medical imaging, one of the key limitations associated with performing DWI for diagnosis is that the DWI imaging in current scanners is only achievable at a lower resolution and lower signal-to-noise ratio compared to the resolution and SNR that can be achieved using other MRI modalities such as T2-weighted imaging (T2w-MRI) using current scanners. This results in DWI images having lower image quality than is desirable.
Current techniques used to improve MRI image quality such as modifications to pulse sequences and acquisitions over a high number of excitations are either not feasible or not practical for use in DWI for medical diagnosis. More advanced techniques used for improving MRI image quality that is possible with DWI is multi-acquisition superresolution imaging, with has been applied to both T2w-MRI [1, 2, 3, 4] and DWI [5,6]. In multi-acquisition superresolution imaging, multiple MRI acquisitions are performed, and the resulting acquisitions are then combined to reconstruct an MRI image with higher effective resolution. However, multi-acquisition superresolution imaging necessitates multiple acquisitions of the same modality with the same settings (which increases scan times), as well as requiring complex imaging modifications and registration algorithms to properly align the multiple acquisitions together so that they can be combined. Another technique that can be used to improve MRI image quality for DWI is to reconstruct MRI images with high resolution from the lower-resolution MRI images using information from coplanar high resolution Mill images acquired of the same subject [7,8]. While this technique does not require multiple acquisitions of the same modality with the same settings, it relies on the additional acquisition of coplanar high resolution MRI images, as well as registration algorithms to properly align the coplanar high resolution MRI images to the low resolution MRI images for each patient acquisition, which can be difficult to achieve reliably and accurately. Another technique that also does not require multiple acquisitions of the same modality with the same settings are interpolation/inpainting methods [9, 10], where the low resolution MRI images are mapped into a higher resolution spatial domain grid and interpolation filters and inpainting methods are used to fill the gaps [9], or mapped into a higher resolution Fourier domain grid and then the high resolution image computed using the Fourier Transform [10]. While such methods improve MRI resolution, they do not improve or bring out tissue detail or contrast. Another technique for improving MRI image quality are methods that specifically remove undesirable “ghost” artifacts [11, 12]. While such methods are useful for ghost cancellation to improve such undesirable artifacts, they in general do not improve tissue detail or contrast in the MRI images. As such, an alternative form of improving image quality of magnetic resonance imaging that gets around these issues is highly desired, particularly for performing DWI for diagnosis of diseases such as cancer, as improving tissue detail and contrast is important for diagnostic purposes.
Therefore, what is needed are further improvements in MRI and DWI image quality utilizing novel imaging technologies and image processing techniques.